In the practice of cryogenic rectification a feed stream, such as feed air, is passed into a cryogenic rectification plant, such as a double column plant, for separation. One or more product streams are withdrawn from the cryogenic rectification plant and recovered. The feed stream flowrate is set to enable production of product at the desired demand rate.
During the course of operation of the cryogenic rectification plant the demand rate for one or more products may change. This necessitates a change in the capacity of the plant wherein the feed flowrate is changed. Unless specific control action is taken to prevent it, a change in the feed flowrate will cause a temporary change in the liquid to vapor (L/V) ratio within one or more of the columns until the system can return to equilibrium or steady state performance. The temporary L/V change is due to a disparity between the manner in which the feed flowrate change alters the vapor rate (V) within the columns compared to how the liquid rate (L) within the columns is altered. This change in the L/V ratio is undesirable because it adversely affects product purity. Accordingly, it is desirable to maintain the L/V ratio at the desired ratio during and after a change in the feed flowrate.
The cryogenic rectification industry has addressed this issue by providing cryogenic rectification plants with liquid storage or holding tanks to change the capacity of a cryogenic rectification plant in a controlled manner by providing liquid to and/or receiving liquid from a column to adjust the L/V ratio within the column. Such systems are effective but entail high capital costs for the tanks and the attendant piping.
Accordingly, it is an object of this invention to provide a method for changing the capacity of a cryogenic rectification plant in a controlled manner without the need for storage or holding tanks to adjust the L/V ratio of a column.